The present invention relates generally to the field of signal processing and particularly to a signal processing circuit for floating signal sources using positive feedback.
Floating source signals are normally processed in order to make effective use of the source signal. Normally, negative feedback systems are used to process floating source signals, to linearize the transfer function, and to stabilize the characteristics of the circuit.
Referring to FIG. 1A, a system diagram for a basic feedback system is shown. The feedback system can be implemented with negative or positive feedback to process floating source signals. The feedback system is comprised of a floating signal source 1 coupled to a forward gain circuit 2 and a feedback gain circuit 3. The forward gain circuit 2 has a forward gain of A and includes an input terminal 2a and an output terminal 2b. The feedback gain circuit 3 has a feedback gain of F and includes an input terminal 3a and an output terminal 3b. The gain of the feedback system is calculated as the ratio of voltage at the output terminal 2b to the voltage at the input terminal 2a of the forward gain circuit 2.
Referring to FIG. 1B, a circuit diagram for a conventional negative feedback circuit is shown. A signal from a floating signal source 4 is coupled to a forward gain circuit 9, which in turn is coupled to a feedback gain circuit 6. The forward gain circuit 9 is comprised of an amplifier 5 with a first input terminal 5a coupled to the floating signal source 4, a second input terminal 5b coupled to the feedback gain circuit 6, and an output terminal 5c coupled to the feedback gain circuit 6. The feedback gain circuit 6 includes a first resistor R17 and a second resistor R28 that are both coupled to the second input terminal 5b of the amplifier 5. The floating signal source 4 produces a voltage Vin. The forward gain circuit 9 has a forward gain of A and the feedback gain circuit 6 has a feedback gain of F.
The gain of the conventional negative feedback circuit is calculated as the ratio of voltage Vout at the output terminal 5c of the amplifier 5, to the voltage Vin produced by the floating signal source 4 at the first input terminal 5a of the amplifier 5. For negative feedback systems, the system gain Vout/Vin is equal to A/(1+FA). The equation for the system gain of a negative feedback system is shown by the following equations and analysis. Referring to FIG. 1B:
F=R2/(R1+R2)xe2x80x83xe2x80x831.
where F is the feedback gain of the feedback gain circuit 6, R1 is the value of the first resistor 7, and R2 is the value of the second resistor 8.
G=Vout*Fxe2x80x83xe2x80x832.
where G is the voltage at point G and Vout is the voltage at the output terminal 5c of the amplifier 5.
Vout=(Vinxe2x88x92G)*Axe2x80x83xe2x80x833.
where Vin is the voltage produced by the floating signal source 4 and A is the the forward gain of the amplifier 5. Therefore, using equations 2 and 3:
Vout=[Vinxe2x88x92(Vout*F)]*A
Vin*A=Vout*(1+FA)
Vout/Vin=A/(1+FA)
The forward gain A is typically much greater than 1 for negative feedback systems, and can often be in the range of 105 or 106. Therefore, the system gain can be approximated as A/FA or 1/F.
If, for example, the feedback gain circuit 6 consists of a resistor divider including the first resistor R17 with a resistance value of 9R and the second resistor R28 with a resistance value of 1R, then the feedback gain F would be equal to 1/10, thereby, producing a system gain of 1/F or 10. By choosing appropriate resistance values, other values for the feedback gain F and the system gain may be obtained.
High gain operational amplifiers are commonly used as the forward gain circuit in negative feedback systems. However, operational amplifiers are complex and slow since a signal must pass through several stages of transistors. The speed of the circuit is degraded with each stage. The speed of operational amplifiers is also degraded by the integrating capacitor used to stabilize the amplifier.
Another disadvantage of negative feedback systems is that they are prone to instability or oscillation. Oscillation will occur if there is a frequency where the open loop gain of the feedback system is greater than one and the signal passing through the feedback system is phase shifted 360 degrees. The open loop gain of the feedback system is determined by the product of the forward gain A and the feedback gain F. In negative feedback systems, there is a chance of oscillation since the forward gain A is almost always much greater than 1, and therefore the open loop gain will almost always be greater than 1.
Also, since the signal inversion of the negative feedback system gives the equivalent of a 180 degree shift and the operational amplifier used in negative feedback systems provides a 90 degree phase shift at all frequencies, if the feedback gain circuit 3 provides an additional 90 degree phase shift, the negative feedback system will oscillate. Although a feedback gain circuit consisting of only resistors will not cause a phase shift, any feedback gain circuit containing a capacitor may cause a 90 degree phase shift in the signal and cause oscillation.
Negative feedback circuits can also contribute a significant amount of noise due to the resistors typically used in negative feedback circuits. Transistors used in amplifiers also contribute noise. A major source of transistor noise can be modeled as a current source between the collector and emitter wherein a larger current contributes more noise.
A final disadvantage of negative feedback systems is that many operational amplifiers have a constant gain-bandwidth product (i.e., the product of the closed loop gain and the closed loop bandwidth equals a constant) and therefore must trade-off a high gain characteristic with a narrower bandwidth.
Positive feedback systems, on the other hand, are usually avoided in signal processing because they will oscillate or latch up if the open loop gain is greater than 1, which makes it difficult to design such systems to produce an adequate gain. This is because positive feedback systems inherently have a 360 degree phase shift.
Therefore, it is an object of this invention to provide a processing circuit for floating source signals that has greater speed, simplicity, stability, reduced noise, and increased bandwidth over the existing methods.
The present invention comprises a positive feedback signal processing circuit having an open loop gain of less than 1 to avoid oscillation. The signal processing circuit includes a floating signal source, a forward gain circuit comprising a low gain amplifier, and a feedback gain circuit comprising a feedback element and a second stage circuit. The floating signal source produces a voltage that is impressed across the feedback element by the feedback system. The feedback element converts the voltage into an output current. The output current is forced through an output current node to the second stage circuit where the output current can be used as a current reference or be further processed (e.g., amplified to a useable level).
The output from the low gain amplifier may be used as a voltage output node that provides a voltage that is an amplification of the voltage produced by the floating signal source. The signal processing circuit may be embedded in other circuits, including additional stages of the signal processing circuit, to create more complex functions and to create another stage of gain.
The signal processing circuit of the present invention has several uses including use as a floating voltage source amplifier, a current amplifier, a voltage source to current source converter, and can also be used for analog signal processing functions (e.g., integration, differentiation, exponentiation, and logarithms). The circuits of the present invention can be manufactured using gallium arsenide (GaAs) metal-semiconductor field-effect transistor (MESFET) processes and bipolar and complementary metal-oxide silicon or BiCMOS processes.
The signal processing circuit of the present invention is built around a high speed, low gain amplifier, providing high speed operation, high bandwidth and low noise. The signal processing circuit of the present invention is also completely stable since it operates with an open loop gain of less than 1, thereby avoiding the possibility of oscillation.